Title of Invention

A METHOD OF PRODUCING ADVANCED HIGH STRENGTH STEEL FOR AUTOMOTIVE APPLICATION AND STEEL PRODUCED THEREOF

Abstract

This invention relates to a method of producing advanced high strength steel for automotive application comprising the steps of: preparing titanium and molybdenum added low carbon steel having selective composition in weight % C - 0.05 - 0.10, Mn - 1.40 - 1.70, Si - 0.10 - 0.20, S - 0.01 max, P - 0.025 max, Al - 0.02 - 0.05, N - 60 ppm max, Ti - 0.080 - 0.10 and Mo - 0.20 - 0.25, through Basic Oxygen Furnace (BOF) - Ladle Furnace (LF) - continuous casting (CC) route; hot rolling cast slab into strips at a finish rolling temperature of 880° C - 910° C and at coiling temperature of 600 - 640° C to obtain a single phase ferrite phase of average grain size < 4 &#956;m to strength the final steel strip / sheet on inter phase precipitation hardening of nano size ferrite from austenite - ferrite transformation; and characterize evaluating the final sheet / strip strength through test evaluation of tensile, charpy impact, high and low cycle fatigue at room temperature, bend properties, microstructure and stretch flangeability.

Full Text

-2- FIELD OF THE INVENTION The subject invention relates to a method of producing advanced high strength steel for automotive application and steel produced there of. More specifically the present invention relates to a method of producing advanced high strength steel for automotive application through deformation of steel sheet of Ti and Mo added low carbon steel encouraging inter-phase precipitation to produce microstructure of fine ferrite. BACKGROUND OF THE INVENTION In the automobile industry there is continuous necessity in development of thin sheet and strips with low weight for optimized use of energy saving criterion for efficient running of automobile engine with low capacity rating with cost saving, fuel saving and material saving criteria. -3- Such automobile sheet / strip has been produced through maintenance of phase transformation characteristic of y phase in Duplex and Triplex steel through multiple course of heat treatment of steel on hot working, tempering, cooling rate for producing steel with high strength and ductility. Such development has been taken course studying behaviours of low alloying constituents along with hot rolling and cold rolling to maintain final microstructure from ferrite-bainite and martensite as double phase or triple phase microstructure on maintenance of two mutually contradictory properties of ductility and high strength with good press forming properties. In the existing practice there is lack of Stretch Flangeability is observed in dual Phase (DP), Transformation Induced Plasticity (TRIP) multiphase steels. The proposed invention has been developed to solve the difficulties of prior art by providing light weight steel sheet for chasis application of automobile on developing a single phase ferritic steel strengthened by precipitation hardening with high strength and excellent ductility coupled with stretch flangeability on obtaining > 800 MPa. -4- DESCRIPTION OF THE INVENTION One objective of the invention is to produce prepare a titanium and molybdenum added low carbon steel having selective composition in weight % C - 0.05 - 0.10, Mn - 1.40 - 1.70, Si - 0.10 - 0.20, S - 0.01 max, P - 0.025 max, Al - 0.02 - 0.05, N - 60 ppm max, T\ - 0.080 - 0.10, Mo - 0.20 - 0.25, through Basic Oxygen Furnace (BOF) - Ladle Furnace (LF) - continuous casting (CC) route. Another object of the invention is to hot roll the cast slab through a six strand Hot Strip Mill (HSM) into 1450 x 7 mm section controlling the Finish Rolling Temperature (FRT) at 880° - 910° C and at coiling temperature (CT) of 600 - 640° C. A further objective of the invention is to obtain a single phase ferrite microstructure of the final steel sheet / strip strengthened by precipitation hardening. -5- Yet another objective of the invention is to achieve high strength high ductile characteristic of the hot rolled strip / sheet of YS 700 MPa, UTS > 800 MPa and elongation >. 20. A still another objective of the invention is to characterize evaluate the final sheet / strip strength through testing of tensile, charpy impact high cycle fatique at room temperature, bend properties and microstructures, low cycle fatique, hole expansion (stretch flangeability). DETAILED DESCRIPTION OF THE INVENTION The selected composition of the continuous cast steel slab for deformation is as in Table 1 below. Table 1, composition of steel investigated (Wt %) c Mn Si S P Al N Ti Mo 0.0. 05-10 11 40-70 0.0. 10-20 0. 01 max 0. 025 max 0.0. 02-05 60ppmmax 0.0. 080-10 00 20-25 -6- Mn, Mo were added for strengthening and to lower the austenite-ferrite transformation temperature so that inter-phase precipitation of nano sizes takes place at lower temperature and precipitates also become more thermally stable. During the course of rolling from higher temperature and also the cooling at run out table (ROT) no precipitation takes place. Mo, Mn and titanium stay in the solution of austenite, which has also higher solubility. Effectively these elements in solid solution lowers the austenite to ferrite transformation temperature to as low as 620 °C. Solubility of these elements in ferrite is very low, hence when transformation takes place Mo and Ti come out of the solution and form mainly carbide precipitates in a rows at austenite-ferrite interface at the transformation front. As these precipitates are forming at very low temperature, they will be fine (1-4 nanometer sizes) and thermally stable. Mo and Ti were added for producing these thermally stable precipitates of nanometer sizes. The heat with above steel composition was processed through BOF-LF-CC route. To obtain low carbon melt with high Mn content, the addition of Si-Mn was done in the ladle during tapping and rest amount was added at Ladle Furnace (LF) as manganese metal. Ferrotitanium and Ferromolybdenum were added in LF after proper degassing. -7- The steel melt was continuously cast in to slab of 1450 mm width and 210 mm thickness with a casting speed of 1.1 m/minute. The slabs were hot rolled into strip of 1450 mm width and 7 mm thickness at a finish rolling temperature (FRT) of 880-910° C and coiling temperature of 600-640° C respectively. Since the phase transformation and precipitation occurred at down coiler during and after coiling the rolling could be carried out at higher FRT and hence the rolling load experienced at the finishing mill was lower than that of any other available high strength steel (TS > 500 MPa). The material was characterized for tensile, bend, charpy impact (RT), high cycle fatigue, and microstructural properties. Table 2 below shows mechanical properties of steel. Table 2: Mechanical Properties of steel YS, MPa UTS, MPa Total Eln, % Uniform Eln, % n value, min 700 min 815 min 20 min 12 min 0.13 -8- Tensile strength more than 850 MPa, total elongation 20% minimum with 12% uniform elongation were easily attained. The results of charpy V-notch impact test are shown in Table 3. The specimens were in sub-standard size 55 x 10 x 5 mm. The impact energy was measured to be 75 Joules at room temperature (RT). Table 3: Impact test results at room temperature (RT) Temperature Impact Properties RT 75 0 The invention is illustrated with the following accompanying drawings in which Figure 1 represents a S - N (stress - No. of cycles) curve generated from high cycle fatique test. Figure 2 shows photographs of closed bend tested specimen. Figure 3 represents a scanned electron micrograph showing the microstructure of the steel under investigation. -9- The high cycle fatigue test was carried out as per ASTM standard (E466) under stress control mode (R = -1, sinusoidal waveform at l0Hz). The S-N curve generated from this test is shown in Figure 1. All the tests were carried out till the fracture of the sample. The data generated from the above test is shown in Table 4. Table 4 showing the Fatigue properties Fatigue Properties Value Fatigue Strength Coefficient 1660 MPa Fatigue Strength exponent -0.0945 Endurance limit, 5 x 106 cycles 420 MPa Fatigue Ratio 0.484 The IT, 2T and closed bend tests of this grade were carried out as per BIS 1599. The picture of the closed bend sample is shown in Figure 2. Not only in IT, 2T bend tested samples but also in the closed bend tested samples, no defects or cracks could be observed at convex portion of steel plate along the bend axis. -10- Figure 3 shows a Scanning electron micrograph of steel under investigation. It can be seen from the micrograph that the microstructures mainly consist of fine ferrite grains. Other phases like cementite and bainite could be observed at a very less extent and in a scattered manner. The average grain size of ferrite was measured to be <. mm.> The invention as illustrated and exemplified with a particular exemplanary embodiment should not be read and construed in a restrictive manner as various modifications, alterations and adaptations are possible within the scope and ambit of the invention as defined in the appended claims. -11- WE CLAIM 1. A method of producing advanced high strength steel for automotive application comprising the steps of: - preparing titanium and molybdenum added low carbon steel having selective composition in weight % C - 0.05 - 0.10, Mn - 1.40 - 1.70, Si - 0.10 - 0.20, S - 0.01 max, P - 0.025 max, Al - 0.02 - 0.05, N - 60 ppm max, Ti - 0.080 - 0.10 and Mo - 0.20 - 0.25, through Basic Oxygen Furnace (BOF) - Ladle Furnace (LF) - continuous casting (CC) route; - hot rolling cast slab into strips at a finish rolling temperature of 880° C - 910° C and at coiling temperature of 600 - 640° C to obtain a single phase ferrite phase of average grain size strengthen the final steel strip / sheet on inter phase precipitation hardening of nano size ferrite from autenite - ferrite transformation; -12- - and characterize evaluating the final sheet / strip strength through test evaluation of tensile, charpy impact, high and low cycle fatigue at room temperature, bend properties, microstructure and stretch flangeability. 2. A method of producing advanced high strength steel for automotive application as claimed in claim 1, wherein high strength high ductility steel strip / sheet of 7 mm thickness is produced having YS 700 MPa, UTS .> 800 MPa, total elongation of > 20 % with 12 % uniform elongation. 3. A method of producing advance high strength steel as claimed in claims 1 and 2, wherein in the steel composition Mo and Ti are added for promoting thermally stable precipitates of ferrite of nanometer size from ferrite solution by forming carbide precipitates in a row at austenite-ferrite interface at the transformation front and Mn, Mo added to enhance strengthening of the steel strip / sheet on lowering of austenite - ferrite transformation temperature as low as 620° C. -13- 4. A method as claimed in claim 3, wherein during the course of rolling from higher temperature and also during cooling at run out table (ROT) no precipitation takes place and Mo, Mn and Ti in solid solution effectively lowers the austenite-ferrite transformation temperature. 5. A method of producing advanced high strength steel as claimed in the preceding claims wherein n value of the resulted strip / sheet is obtained at 0.13 minimum. 6. A method of producing advanced high strength steel as claimed in the preceding claims wherein impact energy of test specimen of 55 x 10 x 5 mm size on charpy V - notch impact test has been obtained as 75 Joules. -14- 7. A method of producing advanced high strength steel as claimed in the preceding claims wherein on high cycle fatigue test according to ASTM Standard (E466) under stress control mode (R = -1, sinusoidal waveform at 10 Hz) on generation of stress amplitude Vs Number of cycles (S - N) curve on the test samples from the resulted sheet / strip steel till the fracture of the samples, endurance limit of the steel as 420 MPa at N = 5 x 106 cycles with fatique ratio of 0.484 is obtained. 8. A method of producing advanced high strength steel as claimed in the preceding claims wherein on carrying out IT, 2T bend test and closed bend test of the samples from the resulted steel strip / sheet according to BIS 1599 standard, no defects or cracks could be observed at the bended portion of the steel strip / sheet along the bend axis. 9. A steel strip / sheet having composition of C - 0.05 - 0.10, Mn - 1.40 - 1.70, Si - 0.10 - 0.20, S - 0.01 max, P - 0.025 max, Al - 0.02 - 0.05, N - 60 ppm max, Ti - 0.080 - 0.10, Mo - 0.20 - 0.25, as produced according to claim 1. -15- 10. A steel as claimed in claim 8, wherein the steel is produced through BOF - LF - CC route on addition of Si - Mn in the ladle during tapping, adding Manganese at ladle furnace (LF) and adding ferrotttanium and ferromolybdenum in the LF after degassing. This invention relates to a method of producing advanced high strength steel for automotive application comprising the steps of: preparing titanium and molybdenum added low carbon steel having selective composition in weight % C - 0.05 - 0.10, Mn - 1.40 - 1.70, Si - 0.10 - 0.20, S - 0.01 max, P - 0.025 max, Al - 0.02 - 0.05, N - 60 ppm max, Ti - 0.080 - 0.10 and Mo - 0.20 - 0.25, through Basic Oxygen Furnace (BOF) - Ladle Furnace (LF) - continuous casting (CC) route; hot rolling cast slab into strips at a finish rolling temperature of 880° C - 910° C and at coiling temperature of 600 - 640° C to obtain a single phase ferrite phase of average grain size phase precipitation hardening of nano size ferrite from austenite - ferrite transformation; and characterize evaluating the final sheet / strip strength through test evaluation of tensile, charpy impact, high and low cycle fatigue at room temperature, bend properties, microstructure and stretch flangeability.

Documents:

00958-kol-2007-abstract.pdf

00958-kol-2007-claims.pdf

00958-kol-2007-correspondence others 1.1.pdf

00958-kol-2007-correspondence others 1.2.pdf

00958-kol-2007-correspondence others.pdf

00958-kol-2007-description complete.pdf

00958-kol-2007-drawings.pdf

00958-kol-2007-form 1 1.1.pdf

00958-kol-2007-form 1.pdf

00958-kol-2007-form 18.pdf

00958-kol-2007-form 2.pdf

00958-kol-2007-form 3.pdf

00958-kol-2007-gpa.pdf

958-KOL-2007-(05-12-2011)-FORM-27.pdf

958-KOL-2007-ABSTRACT.pdf

958-KOL-2007-CLAIMS.pdf

958-KOL-2007-CORRESPONDENCE-1.3.pdf

958-KOL-2007-DESCRIPTION (COMPLETE).pdf

958-KOL-2007-DRAWINGS.pdf

958-KOL-2007-FORM 1-1.2.pdf

958-KOL-2007-FORM 2.pdf

abstract-00958-kol-2007.jpg